1. Field of the Invention
The present invention pertains to pipelines and apparatus for use therewith. In particular, it pertains to apparatus for closing or plugging a section of pipeline in order to repair or work on such. In particular, the present invention pertains to apparatus suitable for isolating a section of pipeline under pressure without the necessity of draining the entire pipeline.
2. Description of the Prior Art
Occasionally it is necessary to work on sections of pipelines for various purposes, e.g., repairing leaks, replacing sections of pipe, installing or replacing valves, connecting branch lines, etc. In the past, it has been necessary to drain the pipeline or a substantial portion thereof in order to repair or perform any other maintenance on the line. This of course resulted in the time-consuming operation of draining the line and transporting the drain fluid to storage facilities. This also results in pump or compressor stations of the pipeline being completely shut down.
Within the past few years devices have been developed to isolate short sections of pipeline, eliminating the necessity for draining substantial lengths thereof. One such device comprises a plug member which is inserted through a lateral opening which has been previously cut in the pipeline by "hot tap" methods. The plug is inserted through the lateral opening and rotated so as to be in axial alignment with the pipeline and is then caused to expand into sealing engagement with the inner walls thereof. The lateral opening which is cut in the pipe for these type devices is substantially the same dimension as the inside diameter of the pipe. While offering certain advantages, this method introduces disadvantages such as the removal of a substantial amount of the pipe by cutting the lateral entry hole, thereby weakening the structural continuity of the pipe itself. Further, these devices are sometimes difficult to insert and remove from the pipeline, creating a possible hazard of permanently blocking the pipeline. Examples of such prior art may be seen in U.S. Pat. Nos. 2,929,410 and 3,285,290.
In other prior art, plugging devices have been provided for traveling inside the pipeline to a predetermined point, propelled by fluid flow within the pipeline. These devices, commonly referred to as "plugging pigs" are launched at some convenient point upstream and travel to a preselected downstream location where small diameter pins, relative to the pipe diameter, are extended into the pipe through valves connected to the pipe and through holes of approximately the same diameter as the pin which have been cut under pressure by "hot tapping" methods. Upon contact with the pin, the plugging pig will stop, at which time a seal or packer element thereon will be expanded sufficiently to establish sealing contact with the inner pipe wall. Various means are used to expand the seal, including the inflation of the seal by fluid connection with an external fluid pressure source through the pin. Examples of such prior art may be seen in U.S. Pat. Nos. 2,929,410 and 3,285,290.
When such prior art plugging pigs are used in pairs to isolate a section of pipeline, it is necessary to establish external fluid bypasses around the first pig, upon arrival at a predetermined pin location, in order to enable the second pig to approach. Such bypasses require at least three hot taps to be made, in addition to the pin device hot taps, in order to establish the desired fluid communication for operation of the two pigs. In addition, as previously stated, a rather complex design may be required to inflate the packer or sealing element from an external pressure source.
In a more recent development a plugging pig has been designed which incorporates an integral fluid bypass valve. Such a valve eliminates the necessity of the external bypasses previously required in the prior art to bring the second pig into position. This also reduces the number of holes in the pipeline and requires less welding. Furthermore, the integral bypass valve may be automatically actuated as the pig approaches its stop pin or restraining device, preventing undue hydraulic shock created by sudden stoppage of the pig. The ingtegral bypass valve is also designed for selective operation externally of the pipeline allowing much flexibility, including the automatic refilling of a newly repaired pipe section. Such a plugging pig and its method of use may be seen in U.S. Pat. No. 3,503,424.